Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A few problems on waves that i need help with

  1. Dec 15, 2004 #1
    I need some help getting started on a few wave and sound problems.

    1.)A closed organ pipe has a fifth harmonic frequency of 1230Hz at 20 degrees Celcius
    a) What is the length of the organ tube?
    b) What is the first harmonic of the pipe?
    c) If one end of the pipe were opened, what would be the first harmonic?

    2.) Consider a sound intensity of 16 x 10 ^-8 W/m^2. At how many times the original distance would the intensity level decrease by 12 db?

    3.) What is the wavelength (in meters) of a wave whose displacement is given by y=o.5 sin (0.20x + 120t), where x and y are in meters and t in seconds?

    Tarzan swings back and forth on a long vine. His friend Jane notices 30 swings in 2.4 minutes.
    a) the frequency of the swing is?
    b)the vine's length is?

    4.) Middle C has a frequency of 262Hz. What is the frequency of C an octave higher?

    5.) The distance from the moon to the earth is 3.8 x 10^5 km.
    a) How long does it take a light beam to travel this distance?
    b) If instead of a vacuum, the space between had air at STP, how long would it take for sound to travel that same distance?

    6.)Spock has landed on a newly discovered planet and is instructed to determine its gravitational strength. He constructs a simple pendulum with a length of 700mm and observes 20 swings in one minute and 16.7 seconds. What does he deduce the "acceleration of gravity" to be from this?

    Thanks for any help starting these!
  2. jcsd
  3. Dec 15, 2004 #2
    A closed organ pipe has two closed ends. An open organ pipe has one open end. The closed end means that this has to be a node or place of 0 amplitude for the standing wave. The open end means that this has to be a point of maximum amplitude. Find the length of tube in which a resonating (standing) waveform will fit using these conditions. The first harmonic is simply the longest standing wave that fits these conditions.

    What is the relationship between intensity and decibels? Look it up and plug it in.

    What is the distance between two sucessive values for x where y = 0 (for a fixed time)? What is the frequency of this wave (ie. what is the time between two successive values for t where y =0 (fixed x)?

    This is just a pendulum. What is the formula for a pendulum in terms of length and period? Look it up and plug it in.

    What is an octave in terms of frequency ratio?

    What is the speed of light in space? in air? (check index of refraction in air and use Snell's law).

    Again just a pendulum problem. Look up the formula for pendulum period in terms of length and solve for 'g'. That is the acceleration due to gravity on the new planet.
  4. Dec 16, 2004 #3
    What happened to Spock's "gravitometer" with "Einsteinian compensators" ? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook